Silicon photodiode optical sensors which convert light to current generally have non-ideal spectral response characteristics due to thin film interference effects of the silicon dioxide layers deposited on the silicon of the sensor during wafer processing. The thin film generates an interference ripple superposed on the photodiode response. The interference ripple is caused by a wavelength dependent variation of the reflection and thus of the transmission. For many applications, the amplitude of the interference ripple is a more important issue than the absolute value of photodiode response.
The reflection for normal incident light of a composite Material1-Material2-Material3 stacked structure where Material2 is a thin film is:R=[R1^2+R2^2+(2*R1*R2*cos(OF))]/[1+R1^2*R2^2+(2*R1*R2*cos(OF))]where:R1=Reflection Factor at Material1-to-Material2 interfaceR1=(N2−N1)/(N2+N1)R2=Reflection Factor at Material2-to-Material3 interfaceR2=(N3−N2)/(N3+N2)OF=Optical FactorOF=(4*PI*N2*T2/W)N1=Refractive Index of Material1N2=Refractive Index of Thin Film Material2N3=Refractive Index of Material3PI=3.1416T2=Thickness of Thin Film Material2W=Optical Wavelength
The transmission thru this stacked structure is (1−R).
A historical approach to minimize reflection for simple photodiodes has been to deposit a thin film which ideally has:N2*T2=Wd/4 (quarter-wavelength thin film)N2=SQRT(N1*N3)whereWd=Design Wavelength
For this ideal thin film, R1=R2 and R=0 at design wavelength. But the reflection at other wavelengths can be as high as (4R1^2)/((1+R1^2)^2). This approach is usually not compatible with standard silicon integrated circuit processing.
Another approach used to achieve certain reflection characteristics is the use of complex stack of multiple layers of thin films with different materials having at least two different refractive indices. But this approach generally requires custom materials and/or processes.
Photodiodes and other optical sensors are now being integrated into complex silicon integrated circuits which have embedded signal processing. Thus the basic thin film is silicon dioxide and is inherent in the wafer processing. Also with multiple interconnect metal layers used in advanced silicon technologies, the silicon dioxide is relatively thick, which results in the interference ripple having amplitude peaks which are closely spaced. In a typical application, the photodiode integrated circuit is encapsulated in a clear plastic package.
An example of a typical structure is shown in FIG. 1:
N1=1.55 for Material1 transparent plastic used for packaging.
N2=1.46 for Material2 Silicon Dioxide at 550 nm.
N3=4.08 for Material3 Silicon at 550 nm.
The refractive indices are assumed to be constant for enclosed examples but actually the refractive indices can be wavelength dependent.
The reflection due to the plastic-oxide-silicon stack is shown in FIG. 2 for T2=4.0 um=4000 nm for the 400-700 nm visible wavelength range. The peak-to-peak amplitude of the interference ripple on reflection is (0.251−0.202)=0.049. The thin film transmission (1−R) would exhibit same variation. A need has thus arisen for an optical sensor with minimal thin film interference ripple.